Causes of Low Bottle Production Rate of Hot Rail Steel Plate (HP295) Used for Welding Gas Cylinders

HP295 were found out , and corresponding solutions were proposed. the HP295 hot-rolled steel plate produced by continuous casting billet, which has good size, stable mechanical properties and excellent welding performance, but the comprehensive bottle forming rate is only 92 %, which is still far from the 99 % required by the user , which affects the use and reduces the product quality. reputation and market share. Through the analysis and research on the reasons of waste products of HP295 bottles, the corresponding improvement measures are put forward.
1 waste type
HP295 finished bottle waste products: 〒 is bottle body defects, such as bottle body delamination, bulging, cracking when pressing, and perforation in water explosion test. The number of such waste products is very small, but the harm is relatively large; Cracks appear when shrinking, and most of the waste products are of this type .
2 Cause Analysis of Bottle Defects
Metallographic examination found that there are a large number of non-metallic inclusions in the layered and bulging parts of the bottle body, and there is an "inner folding" phenomenon; micro-area composition analysis shows that the inclusions contain Ca, Si , Mn , Fe , Na , Al and other elements . Table lo
From the results of point analysis, the inclusions are not homogeneous, and the composition of different parts varies greatly, mainly composed of CaO(CaF 2 ). SiO2.Na 2 O.Al 2 O 3 .MnO.Fe oxides, etc. Mainly composed of complex oxides. Among them, CaO(CaF 2 ) and SiOj have the highest content, accounting for 27.71 % -36.20 % and 21.08 % -27.83 % respectively . This is enough to show that the inclusions come from steel slag, but whether it is from tundish slag or crystallization The powder of the device depends on the content of other oxides; due to Na ?. As high as 0.33 %~ 8.86 %, it is judged to be mold powder. This is because although there is a small amount of Na in steel slag and refractories,
But its oxide content is very low, only the mold powder Na content is higher. . As for the oxides containing a large amount of Mn and Fe , the analysis is considered to be the oxidation product of molten steel.
' Table 1 Electron probe point analysis results/%

analysis point	Na 2 O	AQ	SiQ	FeO, Fe2O3	CaO, CaF 2	MnO
1	0.33	0.51	26.91	4.99	35.27	31.98
2	8.86	17.91	27.83	4.38	28.71	12.30
3	1.41	4.98	21.08	13.58	36.20	22.68

Optical metallographic analysis found that there are a large number of non-metallic inclusions in the crack and near the crack, mainly silicate, AI2O3, and spinel , with a size of 1.5 mm to 3 mm; The particles are spherical inclusions, the matrix is calcium aluminate, and it is also mixed with AI 2 O 3 and granular MnO, which are mainly the products of deoxidation and secondary oxidation.
It can be seen that the defects of the bottle body are mainly caused by the impure steel and the presence of a large number of non-metallic inclusions.
3 Sources and reduction measures of non-metallic inclusions
3.1 Source
O and N in front of nitrogen, after nitrogen and in tundish were measured , and Figure 1 was drawn according to the results . Since the content of O and N increases after the molten steel enters the tundish from the ladle , it means that the molten steel is oxidized when it flows from the ladle to the tundish. The main reasons for the secondary oxidation of molten steel are low self-casting rate, lax coverage of covering agent, and exposure of high-temperature molten steel to the air.

In order to quantitatively analyze the total amount of oxygen in steel, and to distinguish the types and contents of various oxides in detail, electrolytically separate the © 15 mm x 120 mm continuous casting slab sample to measure the total amount of inclusions, and use atomic absorption method and spectrophotometry to determine the oxygen content of each oxide. The results are shown in Table 2 . The content of AI2O3 is the highest, accounting for 67 % of the total inclusions , indicating that the oxide inclusions are mainly derived from the deoxidized products that do not float up and stay in the molten steel.
Table 2 Electrolytic inclusion results/%

 

furnace number	CaO	MgO	SiOz	AIQ3	FeO	MnO	total	Oxide always	Li 2 . 3/ total
24958	4	5	3.7	40	2.7	0.24	55.64	60	67
15158	2.8	11	5.6	30	not detected	not detected	49.4	78	38
15072	1-8	3.4	4.0	19	3.6	not detected	31.8	62	31
24880	12	17	3.2	19	6.5	1.1	58.8	79	twenty four
15071	16	9	0.41	53	0.41	0.2	79.02	85	62
-15068	19	13	1.1	12	not detected	1.0	46.1	57	twenty one
15157	16	6.1	0.54	29	not detected	1.1	52.7	58	50

From the above analysis, it can be known that the non-metallic inclusions come from mold slag volume, molten steel secondary oxidation and deoxidation products that do not float up and stay in the steel.
3.2 Measures

1. In order to avoid mold slag entrainment, the casting table operation should be strengthened to reduce slag entrainment caused by mold liquid level fluctuations.
2. In order to reduce the secondary oxidation of molten steel, the self-pouring rate should be increased, and the whole process of protective pouring should be implemented.
3. In order to promote the deoxidation products in the steel to float up, the CaSi wire is refined and fed, and the inclusions are denatured and promoted to float up. 3.3 Effect

By taking the above measures, the inclusions in the steel plate are reduced from the original 2.0-3.0 level to 0.5 level; the maximum size of the as-cast inclusions is reduced from 162 to 70 "m, and the number of inclusions with a size greater than 50 is also greatly reduced. And there is no waste product of bottle defects.
4 Causes and countermeasures of cracks when the head is stamped or shrinked
There are two types of situations found in metallographic inspection cracks: one is that there are a large number of non-metallic inclusions in the cracks, and the reasons and countermeasures for the inclusions have been mentioned above; the other is that the content of inclusions in the cracks is not high, but the microstructure is Abnormal structure (acicular ferrite + granular bainite), this kind of situation mostly occurs within 40 m of capuchin , and this kind of situation is mainly analyzed here.
4.1 Cause Analysis
Cracking is caused by the high yield strength of the steel plate used for stamping, which leads to excessive residual stress inside the stamping part, which causes the redistribution of stress concentration during the machining process, and makes the local residual stress exceed the fracture strength of the steel.
distance between the end stand of the finishing mill of Benxi Iron and Steel Co., Ltd. and the cooling water in section I is only 10.5 m, and the strip begins to cool after about 1 s of finishing rolling . Due to insufficient recrystallization and fine grains, the yield strength is relatively high. In addition, the generated granular bainite also increases the strength. This type of defect mostly occurs within 40 m of the steel coil tail because the forming spokes of the plate rolling machine are immediately held tightly after the steel coil is rolled, and at the same time the shaping water is opened to spray a large amount of cooling water on the outer ring of the steel coil, resulting in the outer ring of the steel coil being water quenched. , resulting in a 30-40 MPa increase in the yield strength of the curly tail compared to other parts . 4.2 Measures

1. In view of the defects of the cooling process, the laminar flow cooling is given priority to open the n- stage water, and when it is insufficient, add the last several groups of water in the I -stage, so that the steel has a certain recrystallization time after final rolling, promotes the grain growth and reduces the yield strength.
2. In order to reduce the influence of the shaping water of the coiling machine on the strength of the tail of the steel coil, when the steel strip is 40 m away from the coiling machine , the setting water of the coiling machine is turned off to reduce the temperature drop of the outer layer of the coil and weaken the water quenching effect, so that Capuchin organization and performance are improved.

4.3 Effect
Table 3 shows the performance comparison before and after taking measures. It can be seen that the yield strength, tensile strength and yield strength ratio have been greatly reduced, and the elongation rate has been significantly increased. This is beneficial to improving the stamping formability and punching rate. Greatly reduce the range of fluctuations in the longitudinal mechanical properties of steel coils.

Table 3 The force-energy comparison results before and after taking measures

date	/MPa	% /MPa	/%	6 Buddha
forward	360 ~ 470	360 ~ 470	28 ~ 35	0.73-0.80
	392	521	31.5	0.75
back	310-390	470-520	33 ~ 40	0.65-0.75
	342	487	37	0.70
mean difference	-50	-34	+ 5.5	-0.05

1. Impure steel, a large number of non-metallic inclusions, and high strength at the end of the steel strip are the main reasons for the low comprehensive bottle production rate of Benxi Steel's HP295 .
2. A large number of non-metallic inclusions mainly come from mold powder inclusions, deoxidation products and secondary oxidation products.
3. The high strength of the tail of the steel coil is mainly caused by improper cooling process and improper control of the shaping water of the coiling machine.
4. Under the existing equipment conditions, by taking measures such as reducing the liquid level fluctuation of the crystallizer, feeding CaSi wire, increasing the self-casting rate, protecting the pouring in the whole process, optimizing the cooling process, and closing the shaping water of the plate rolling machine in a timely manner, the HP295 of Benxi Iron and Steel Co., Ltd. has been greatly improved . The comprehensive bottle-making rate can reach more than 99 %, which can meet the strict requirements of users on the bottle-making rate.

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